The broad long-term objectives of our research on the transmission and control of Schistosoma japonicum infections in China have been to develop methods for the design of site-specific control programs to achieve the sustainable cessation of transmission. The recent re-emergence of transmission in formerly controlled counties has prompted this proposal, in which we extend our objectives to include an understanding of the determinants of re-emergence, which will inform the design of surveillance programs for the early detection and minimization of the scale and impact of re-emergence. The aims of our research are to 1. Test the hypothesis that re-emergence is driven by two village-level factors: internal potential (a village's inherent ability to support transmission that includes snail density, human and animal hosts, and agricultural practices) and connectivity (factors related to the introduction of parasites from endemic areas), which can be obtained from existing historical records and remote sensing;2. Using a case-control study, test the hypothesis that individual-level within-village and outside-village water contact is predictive of the initial infections in re-emergent villages;and 3. Utilize a stochastic model of re-emergent transmission to explore the effect of chance events on the design of an effective surveillance system. Relevance to public health: This research is relevant to public health, in that it will increase our knowledge of the factors related to infectious disease re-emergence. Through this understanding, better surveillance approaches can be developed to protect public health.